When Harmless Bacteria Became Flesh-eating Monsters

New research indicates that one of the most virulent flesh-eating bacteria went rogue in 1983 after undergoing a series of genetic changes.

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Most of us have heard of flesh-eating bacteria, which attack healthy flesh at a fearsome rate. But few are aware that these bacteria were once benign, before taking a murderous turn.

Now, scientists at the have Methodist Hospital Research Institute in Houston, Texas, have tracked the evolutionary history of one of the bacteria and pinpointed precisely when it became a mass murderer, according to an article on The Conversation website.

The research, published in the Proceedings of the National Academy of Sciences, focused on a flesh-eater called GAS, or Group A β-hemolytic streptococcus, a highly infective bacteria. Apart from causing flesh-eating disease, GAS is also responsible for a range of less harmful infections.

It affects more than 600m people every year, and causes an estimated 500,000 deaths.

“This is the first time we have been able to pull back the curtain to reveal the mysterious processes that gives rise to a virulent pathogen,” said lead researcher James Musser.

The GAS bacteria first began to affect humans in the 1980s. To find out where it came from and why it turned deadly, Musser and his team analyzed bacterial genetic data from across the world. A total of about 3,600 streptococcus strains were collected and their genomes recorded.

It revealed that a series of distinct genetic events turned this bacteria rogue.
First, foreign DNA moved into the original harmless streptococcus by horizontal gene transfer – a phenomenon that is common among bacteria. Such DNA is often provided by bacteriophages, viruses that specifically target bacteria.

In the case of GAS, the foreign DNA that was incorporated in the host’s genome allowed the streptococcus cell to produce two harmful toxins. A further mutation to one of these toxin genes made it even more virulent.

Then a second horizontal gene transfer provided an additional set of genes, allowing the pathogen to produce proteins that suppress the immune system of those infected, making the infection worse. GAS became a serial killer.

Using statistical modeling, Musser was able to date the last genetic change, which turned GAS into a highly virulent bacteria, to 1983.

“The date we deduced coincided with numerous mentions of streptococcus epidemics in the literature,” Musser said.

Since 1983, there have been several outbreaks of streptococcus infections across the world, becoming the inter-continental epidemic that it is today. The symptoms range from pharyngitis to the flesh-eating disease, necrotizing fasciitis.

“In the short term, this discovery will help us determine the pattern of genetic change within a bacteria, and may help us work out how often bacterial vaccines need to be updated,” Musser said. “In the long term, this technique may have an important predictive application – we may be able to nip epidemics in the bud before they even start.”